I’ve checked … presumably the bottom article is the one: https://www.anodeoutlet.co.uk/vessel-types/.
I’m pretty sure that my supplementary anode is connected to the saildrive housing (not the engine) as recommended. Will check next time I’m at the boat.
What I do notice is that the big ring anode on the saildrive wears very little. But the split-ring one on the prop (I have a Brunton’s Varifold) gets almost completely eaten away in under a year. It could be that the supplementary anode IS working, but that it is protecting the big ring anode, not the little split-ring one.